|
|
| Line 3: |
Line 3: |
|
| |
|
| ==Introduction== | | ==Introduction== |
| The single-stranded DNA-binding protein (SSB) of Escherichia coli is involved in all aspects of DNA metabolism: replication, repair, and recombination.
| |
| In solution, the protein exists as a homotetramer of 18,843-kilodalton subunits. As it binds tightly and cooperatively to single-stranded DNA, it has become
| |
| a prototypic model protein for studying protein-nucleic acid interactions. The sequences of the gene and protein are known, and the functional domains of
| |
| subunit interaction, DNA binding, and protein-protein interactions have been probed by structure-function analyses of various mutations. The ssb gene has
| |
| three promoters, one of which is inducible because it lies only two nucleotides from the LexA-binding site of the adjacent uvrA gene. Induction of the SOS
| |
| response, however, does not lead to significant increases in SSB levels. The binding protein has several functions in DNA replication, including enhancement
| |
| of helix destabilization by DNA helicases, prevention of reannealing of the single strands and protection from nuclease digestion, organization and stabilization
| |
| of replication origins, primosome assembly, priming specificity, enhancement of replication fidelity, enhancement of polymerase processivity, and promotion
| |
| of polymerase binding to the template. E. coli SSB is required for methyl-directed mismatch repair, induction of the SOS response, and recombinational repair.
| |
| During recombination, SSB interacts with the RecBCD enzyme to find Chi sites, promotes binding of RecA protein, and promotes strand uptake.
| |
|
| |
|
| Copies of SSB bind to the unwound DNA strands, keeping the strands separated so that both strands can serve as templates. | | Copies of SSB bind to the unwound DNA strands, keeping the strands separated so that both strands can serve as templates. |